The disclosure relates to methods of logging wells for gas capped oil reservoirs with a known mineral composition of constituent rocks, that is for oil and gas deposits, in particular, to methods of determining characteristics of a gas-oil transition zone, such as the distribution of gas and oil saturations along a wellbore and a gas-oil capillary pressure.
A gas-oil contact (GOC) is an imaginary surface that separates oil and gas in an oil reservoir, where gas is in a free state and forms a gas cap. The surface of the gas-oil contact is conditional as there is a transition zone of mixed oil-gas saturation between the gas and oil parts of the deposit. Determination of position of the gas-oil contact in geological simulation faces known problems, which are associated with the transition zone structure. In most cases, GOC position is determined from results of fluid inflow tests. In analysis of logging data GOC can be also identified from the abrupt change in the content of hydrogen.
However, there are deposits where geological structure does not allow detecting GOC in the form of a clearly defined surface because there are continuous variations in oil and gas saturation in depth. These cases suggest a gas-oil transition zone. The transition zone may extend in depth at distinguishable distances (>1 m). In a region of the transition zone a continuous vertical distribution of gas and oil, which are in thermodynamic and gravitational equilibrium, is established. Compositions, pressures and saturations of gas and oil phases are continuously varying along the geological section of the well in accordance with the equilibrium conditions.
Deposits with a long gas-oil transition zone require detailed information on the structure of this region for correct evaluation of oil and gas reserves and for a reasonable choice of development strategy.
Currently no methods exist in the practice of exploration of oil and gas deposits for determining the parameters of long gas and oil zones (distribution of gas and oil saturations along the wellbore and gas-oil capillary pressure) taking into account conditions of compositional “gas-oil” phase equilibrium.